Once More Into the Breach...
Sure. This is doable. There are some caveats and considerations. So, let's work though them so you can make an informed choice.
Firstly, retrofitting a console is not going to be a money-saving route to owning a Hauptwerk console. You may, by a stroke of good luck, save money but don't start with saving money as the premise for your project. (You can try but you're better to play the lottery instead... just sayin') If you are confident that you will save money by retrofitting a console, it's because you actually know what you're doing and probably don't need to be reading this article.
If you are going to retrofit a console, do it because you love that particular console (it has fantastic ivory keyboards or astonishingly good wood work) or because you love to take on projects like this and you're going to spend the time and money to do it right. You retrofit the console as a labour of love.
Secondly, in almost all cases, you should just gut the console of the existing tone generation and combination action. In rare cases, you can reuse the combination action. However, it's hard to find schematics for most consoles and this should be left for someone with an electronics background. Scanner board manufacturers can help you understand their own equipment. It's very difficult to explain to you what's inside your own organ and how to hook it up from a distance. Figuring out how your console works is your job. That's why it's often better to just gut the console.
So, does it make sense to convert your console? Lets start with an inventory of you:
Are you a techie? Can you trace electrical signals? Read schematics? Solder wires?
There have never been any standardization to how the insides of consoles are constructed or wired. So, if you plan to undertake the job of rewiring the existing components (e.g stops, pistons, keyboards, pedalboards, expression shoes etc.), you're going to need to be able to understand how they are currently wired and be able to move that over to the system employed by the MIDI scanners you choose to use. Doing so may mean using a soldering iron and/or a multi-meter. If the prospect of this is unsettling, then you should probably considering some other options.
If the thought of this makes you say "well, I could learn it. How hard could it be?", then budget some money in for hiring someone to help you troubleshoot. If you don't end up needing someone, then it's like free cash. If you do need someone, you won't feel like it's a never ending money pit you've gotten yourself into... or have to go and justify spending extra money to a dubious spouse.
How are your woodworking skills?
If you end up replacing certain components (e.g keyboards) or adding new components (e.g extra expression shoes, keyboard, pistons etc), you're going to need to do some wood working. What is your skill level with woodworking?
Get a count of everything that you wish to hook into MIDI scanner boards.
How closely do the counts match the largest/favourite sample set that you plan to own? If the numbers are radically different, you may wish to reconsider adding MIDI to this console. Chances are that it will be a challenge to play your favourite sample set and that's not fun. It would be a shame to do a lot of work and then have the console not work as well as you'd like it to.
Unless your keyboards are particularly spectacular for some reason, it may be simpler for you to swap them out for ones that are already MIDI capable that may even have pistons already integrated. This can save you a lot of time and energy and still allow you to keep a beloved console with only a minimal amount of wood work.
Cost-wise, if you are having to pay for a scanner board, and someone to wire your keyboards, the cost can come pretty close to that of a brand new tracker touch keyboard.
One area where retrofitting a console actually does save you money easily is to reuse a pedalboard which is in good shape. In lots of cases, even if you decide not to retrofit the entire console, it's worth it to have someone add MIDI capability to your pedalboard.
The same goes for expression shoes providing that the shoe is already hooked up to a 3K Ohm to 10K Ohm linear potentiometer. Those potentiometers can then plug directly into many of the MIDI scanners on the market. They will also plug directly into all of our MIDIWorks.ca keyboards.
Getting the keyboards, pedalboard, shoes and pistons sorted out is a great step. With the addition of a touchscreen or two, you have all you need to get Hauptwerk running from your console.
If you are hoping to make the stops on your console work, that is also possible. However, draw knobs and tabs are notoriously difficult and clumsy to label for multiple sample sets. Very few of the sample sets have the same spec and stop layout and they are different enough that a significant proportion of the stops won't match up from set to set. If all you ever want to play are different Cavaillé-Coll organs, you might be able to get away with doing this in a classy manner. However, playing only one kind of organ is probably not why you bought Hauptwerk in the first place.
On the other hand, if you plan to create a console that only ever runs a single sample set, then, by all means, go for it! I know more than a few people who recreated specific consoles to match a particular sample set and they're super happy.
One exception to this "non-standard" argument is couplers. Not only are couplers, cctave 16, octave 4 and other similar controls relatively standard between instruments, if they don't happen to exist on the instrument, you can always use Hauptwerk's native couplers and map those physical devices to the virtual ones. So, if you have a row of tabs on the nameboard, you could convert them to a row of couplers without any conflict.
Note that when you are planning to hook up draw knobs or tabs, there are two things you need to attend to. There will need to be something that scans the switch (on/off) and there will need to be something else that drives the light or solenoid. Sometimes, as with our MDKC, they both exist in the same device. Sometimes they don't. Make sure you check that you have both scanning and drivers before you order.
If you are trying to wire up lighted rocker tabs, you need to check how they have wired the switches. Instead of wiring them with an on switch and an off switch, some companies combine the switches and make it toggle on and off. Sometimes, it's just easier to swap out your lighted rocker tabs for ones that are already MIDI capable instead of figuring out the wiring.
As long as you're looking at converting a console, as it is, for the love of the console, and not as a cost-saving option, go for it. The exception to the cost argument would be for those who really know what they're doing with electronics - and more specifically organ electronics.
The compromise will be to reuse the shell, the pedalboard (which is worth keeping and modifying), and sometimes the swell shoes but and replacing keyboards and other innards with MIDI capable equivalents.
Reusing traditional tabs or drawknobs for stop controls is usually inadvisable for use with multiple sample sets. However, converting some of them to control couplers is certainly a possibility.
As always, I hope this helps. If you have specific questions, don't hesitate to contact me at firstname.lastname@example.org